docs: def regex_opt_inner(strings, open_paren):
      """Return a regex that matches any string in the sorted list of strings."""
      close_paren = open_paren and ')' or ''
      # print strings, repr(open_paren)
      if not strings:
          # print '-> nothing left'
          return ''
      first = strings[0]
      if len(strings) == 1:
          # print '-> only 1 string'
          return open_paren + escape(first) + close_paren
      if not first:
          # print '-> first string empty'
          return open_paren + regex_opt_inner(strings[1:], '(?:') \
              + '?' + close_paren
      if len(first) == 1:
          # multiple one-char strings? make a charset
          oneletter = []
          rest = []
          for s in strings:
              if len(s) == 1:
                  oneletter.append(s)
              else:
                  rest.append(s)
          if len(oneletter) > 1:  # do we have more than one oneletter string?
              if rest:
                  # print '-> 1-character + rest'
                  return open_paren + regex_opt_inner(rest, '') + '|' \
                      + make_charset(oneletter) + close_paren
              # print '-> only 1-character'
              return open_paren + make_charset(oneletter) + close_paren
      prefix = commonprefix(strings)
      if prefix:
          plen = len(prefix)
          # we have a prefix for all strings
          # print '-> prefix:', prefix
          return open_paren + escape(prefix) \
              + regex_opt_inner([s[plen:] for s in strings], '(?:') \
              + close_paren
      # is there a suffix?
      strings_rev = [s[::-1] for s in strings]
      suffix = commonprefix(strings_rev)
      if suffix:
          slen = len(suffix)
          # print '-> suffix:', suffix[::-1]
          return open_paren \
              + regex_opt_inner(sorted(s[:-slen] for s in strings), '(?:') \
              + escape(suffix[::-1]) + close_paren
      # recurse on common 1-string prefixes
      # print '-> last resort'
      return open_paren + \
          '|'.join(regex_opt_inner(list(group[1]), '')
                   for group in groupby(strings, lambda s: s[0] == first[0])) \
          + close_paren